The present invention relates to a control device for a hoist mechanism of a crane, preferably for a crane with a jib which can be varied in length and be luffed, with a hydraulic motor driving the hoist mechanism drum the motor forms a closed hydraulic circuit together with a hydraulic pump driven by a motor, preferably by a diesel engine, and with a locking brake for the hoist mechanism drum.
The hydraulic pump and the hydraulic motor, which are both in a closed hydraulic circuit, have a leak which can, for example, amount to two to twenty ltr./min. During the hoisting operation this leak is not particularly noticeable, as the crane operator will always move the control lever in such a way that the load is held or lifted or lowered at the desired speed, without the crane operator noticing that the inevitable leak is always being taken into account with the corresponding control. However, during crane operation it is also necessary to resume a lifting or lowering of the load after the locking brake has been released. The smooth resumption of holding, lifting or lowering of the load after the release of the locking brake would, however, only be possible if the retaining moment applied by the hydraulic motor after the release of the locking brake corresponds exactly to the moment of the hoist mechanism drum. If the locking brake is activated temporarily while the load is air-borne, due to the inevitable leakage after the release of the locking brake the pressure in the high-pressure line of the hydraulic circuit no longer corresponds exactly to the pressure by which the hydraulic motor held the air-borne load before the snapping in of the locking brake so that on the release of the locking brake jerking is inevitable due to the current pressure in the high-pressure line of the hydraulic circuit which deviates from the pressure required by the hydraulic motor.